I suggest you ...

Award credits by word count rather than by recording.

Hi! New user, love the idea--just a quick suggestion: As you know, people come here to place requests, and placing their requests requires credits. Since any recording is worth one credit--no lmatter how long or short--users will naturally do only low-word-count recordings. If credit were based on word count, say 50 words equals a credit, then this would motivate users to tackle the longer recordings. As things stand, I suspect that my four-hundred-word audio request may languish in the queue forever. Thanks!

Have you tried breaking up the recording into shorter segments? It's pretty easy to do with Audacity!
The thing is, it takes between four to eight minutes to do one minute of audio recording - if you have a good typing speed. People don't want to spend two hours transcribing a twenty minute youtube video, no matter how many credits they get for it

Expecting a total stranger to translate a long essay for you for free is expecting a lot, with or without "credits". That said there should probably be a weighted system for recordings that are longer.